Like other MIME-aware MTAs, Postfix downgrades 8bit body content
to 7bit when a remote SMTP server does not announce 8BITMIME support.
If DKIM signatures must survive transmission to servers that don't
announce 8BITMIME, I recommend to downgrade before signing (for
example, specify "-o smtp_discard_ehlo_keywords=8bitmime,silent-discard"
for an SMTP client that delivers to a null filter or to amavisd).
You could specify "disable_mime_output_conversion = no" locally,
but that won't prevent remote MTAs from doing the 8bit to 7bit
downgrade and breaking DKIM signatures.
You prevent Postfix from adding From/Date/Message-ID headers, and
from rewriting addresses in headers, by setting the -G flag for the
sendmail command, and by setting local_header_rewrite_clients to
empty (by default these modifications are enabled only for submissions
by local processes and from clients in local networks).
Otherwise, Postfix should not modify a well-formed email message.
For example, if the address in a from/to/etc. header is not modified,
then the header is passed through unchanged regardless of the content
of mailbox display names, comments, etc.
However, there are no guarantees for out-of-spec email and for some
obsolete forms. For example, Postfix will fold lines > 998 (not
including the <CR><LF>) by inserting <CR><LF><SPACE> to minimize
damage to MIME-formatted messages, will truncate (multiline) headers
that contain >102400 bytes or >10240 tokens, and will replace the
obsolete form "headername :" with the normal form "headername:"
because many things in Postfix expect the normal form.
Wietse
